Casting from double to decimal rounds columns in Scala Spark

Question

I have a dataframe that looks like this when I print it using df.show():

|---id---|----val_dbl----|
|       1|    1.378332E-7|
|       2|    2.234551E-7|
|       3|        4.03E-7|
|--------|---------------|

with the schema

df.printSchema()
|-- id: integer
|-- val_dbl: double

Then I cast val_dbl to decimal:

df.withColumn('val_dec', col('val_dbl').cast(DecimalType(18,7))

But then I get this when I print:

|---id---|----val_dec----|
|       1|        1.38E-7|
|       2|        2.23E-7|
|       3|        4.03E-7|
|--------|---------------|

|-- id: integer
|-- val_dec: decimal(18,7)

But it should look like:

|---id---|----val_dec----|
|       1|    1.378332E-7|
|       2|    2.234551E-7|
|       3|    4.030000E-7|
|--------|---------------|

|-- id: integer
|-- val_dec: decimal(18,7)

How come it isn't printing out the full value? Is it actually rounding, or is this just for convenience? And how can I print the full decimal values? I can't imagine it is actually rounding.